Solid-state energy-responsive luminescent device

ABSTRACT

A solid-state energy-responsive luminescent device comprising an electro-luminescent element which is excited by AC voltage and has been endowed with resistivity and a photoconductive element whose photoconductive sensitivity is controlled by superimposing a DC voltage on the AC operating voltage; an AC voltage and a DC voltage superimposed on said AC voltage being applied across said two elements; said device being constituted so that DC voltage distributed to the photoconductive element decreases corresponding to decrease in the resistance of the photoconductive element relating to excitation by incident energy and that the AC photoconductive sensitivity of the photoconductive element is controlled through the DC voltage in response to the intensity of the incident energy.

I United States Patent 1111 ,5

[72] Inventors Kazunobu Tanaka 2,988,646 6/1961 Wolfe et a1. 250/213Kawasaki-shi, 3,217,168 11/1965 Kohashi 250/213 Tadao Kohaghi,Yokohamadapan 3,300,645 1/1967 Winslow... 250/213 [211 App]. No. 707,7133,358,185 12/1967 Lally 313/108BX [221 Fi ed F b- 3. 9 OTHER REFERENCES[45] Patented l9 1 Thornton ac-dc Electroluminesce Ph ysrcal Rev1ew, 13Assignee gsztlsgsljfi lnd'mrlal vol 1 13, Number 5, March 1, 1959, pp. 1187- 90 250/213 8 a g Japan Primary Examiner-James W. Lawrence [32]Priority Feb. 24, 1967 Assistant Examiner-C. M. l eedom [33] JapanAttorneyStevens,Dav1s, Miller & Mosher [31] 42/1200 5 soups- ENERGYRESPONSIVE ABTRACTz solid-state energy-responsive luminescentLUMINESCENT DEVICE device comprlsmg an electro-lummescent element which15 7 Claims, 3 Drawing Figs excited by AC voltage and has been endowedwith resistivity and a photoconductlve element whose photoconductivesen- [52] (I Y 250/2133 sitivity is controlled by superimposing a DCvoltage on the AC 313/ 108; 31 operating voltage; an AC voltage and a DCvoltage superim- [51) Cl H013 31/50 posed on said AC voltage beingapplied across said two ele- [50] Field ofSearch 250/213, mems; saiddevice being constituted so that DC voltage i 833mg 313/94 108A 1083tributed to the hotoconductive element decreases cord'td 'th 't fth ht drespon mg 0 ecrease 1n e resrs ance o e p o ocon uc- 1561 g gzxsgg stive element relating to excitation by incident energy and that2,905,830 9/1959 Kazan 2,272,692 2/l 9 6 l Thornton the ACphotoconductive sensitivity of the photoconductive 7 element iscontrolled through the DC voltage in response to the intensity of theincident energy.

SEW/CONDUCT IVE EL LAYER SEMICOND LIGHT PERV/OUS ELECTRODE UCT/VEREFLECT/NG LA YER IMPERV/OUS SEMICUNOUCfll/E L A YER SOLID-STATEENERGY-RESPONSIVE LUMINESCENT DEVICE This invention relates to asolid-state energy-responsive luminescent device in which input energysignal is converted or amplified and displayed through a solid-stateimage plate which consists of a combination of photoconductive elementand electroluminescent element and is connected to a power source. Thisinvention is intended to provide a wide range of controllability of thecontrast, that is, gamma value, an improved characteristics in the lowinput energy range, and an extended input energy latitude (that is, aneffective operating range of the input energy signal), by givingappropriate resistive impedances to respective appropriate elements outof constituents of said solid-state image plate, supplying an AC voltagesuperimposed with a DC voltage as the operating voltage and controllingthe magnitude of this DC voltage. In the conventional solid-state imageplates consisting of a combination of photoconductive element andelectroluminescent element, in which the luminescence of theelectroluminescent element is electrically controlled by variation inthe AC impedance of the photoconductive element in response to the inputenergy signal, the AC impedance of the photoconductive element dependson capacitive impedance of the photoconductive element determined by itsgeometrical structure in low input energy range. From this reason andbecause the photoconductivity is essentially less sensitive when an ACvoltage is being imposed on it, the rate of variation in the ACimpedance is extremely low in the conventional image plates. Thus, theconventional image plates, partly because of the photoelectricalnonlinearity of the electroluminescent element, has a low sensitivityand a high gamma value, and accordingly, a very limited input energylatitude. According to this invention, the electrolumine element of thesolid-state image plate and appropriate intermediate elements interposedbetween said electroluminescent element and the photoconductive elementare endowed with resistive impedances, and the photoconductive elementis of such property that its photoconductive sensitivity under an ACoperating voltage is controllably increased by superimposing a DCvoltage onto the AC voltage. In the operation, the DC voltage issuperimposed on the AC voltage applied across the electroluminescentelement and the photoconductive element, and by controlling the value ofsaid DC voltage are attained an improved characteristics in low inputenergy range, a wide controllable range of gamma value and an extendedinput energy latitude.

Now, this invention will be described in connection with an embodiment,referring to the attached drawings, in which:

FIG. 1 shows an equivalent circuit for explaining the principle of thisinvention;

FIG. 2 is a schematic diagram showing a portion of the solid-state imageplate embodying this invention; and

FIG. 3 shows characteristic. curves concerning an embodiment of thisinvention.

Referring to FIG. 1 which is an equivalent circuit of a solidstate imageplace of DC controlled type for displaying a positive output image, theelectroluminescent element and the photoconductive element used being ofthe form of layer, marking Cp indicates the capacitive component of thephotoconductive layer (hereafter, referred to as a PC layer), Rp theresistive component of the PC layer, Ce the capacitive component of theelectroluminescent layer (hereafter, referred to as an EL layer), and Rethe resistive component of the EL layer. Value of Re is selected to beappropriately lower than the dark resistance of Rp. Mark Va indicatesthe operating AC voltage, Vb the variable DC voltage, L the incidentenergy signal and L: the output light. If Vb is zero volt, the circuitis equivalent to a conventional image plate of positive image type.Since the AC impedance due to the capacitance Cp determined by thegeometrical structure is more dominant in the low input energy rangethan the AC photoconductive sensitivity of the PC layer, thecharacteristics of the output light intensity vs the input energy signalis not satisfactory, and

accordingly, the input energy latitude is not sufficiently broad andfurther, the gamma value is considerably high. However, a DC voltage Vbis applied; in a closed DC circuit composed by the resistive componentRp of the PC layer and the component Re of the EL layer, DC voltage Vbpdetermined by said resistive components Rp and Re is applied to the PClayer, being superimposed on AC voltage Vap. In this state, assumingthat the input energy signal is very weak, the resistive component Rp ofthe PC layer is very high in comparison with the component Re, renderingthe DC voltage Vbp nearly equal to the voltage Vb, thus a very high DCvoltage being superimposed on the AC voltage Vap. A satisfactory PClayer of which the photoconductive sensitivity under AC voltage can becontrollably increased by the superimposition of a DC voltage is the onecontaining powder of photoconductive material.

Generally, AC photoconductive sensitivity of photoconductive powderincreases nonlinearly with the increase of the superimposed DC voltage.Therefore, the characteristics is remarkably improved in the low inputenergy range where the proportion of the superimposed DC voltage ishigh. While, with the increase of the input energy signal intensity, theresistive component Rp of the PC layer decreases, accompanied by thedecrease of the component DC voltage Vbp, thus diminishing the improvingeffect of the DC voltage to the AC photoconductive sensitivity of the PClayer. Upon the intent energy reaching a high intensity range, theresistive component Rp of the PC layer decreases to a value negligiblein comparison with the resistive component Re of the EL layer and thecomponent DC voltage Vbp of the PC layer becomes nearly zero. Therefore,the intensity of the output light L in this input range becomesequivalent in the value to that of the conventional solid-state imageplate in which the voltage Vb is zero. That is, whereas thecharacteristics representing the intensity of output light vs theintensity of input energy signal in a high input range is almostequivalent to that of the conventional solid-state image plate ofpositive image type when the DC voltage is zero, the similarcharacteristics in a low input range is remarkably improved by theeffect of the component DC voltage Vbp imposed upon the PC layer byapplying the DC voltage Vb to the device, the effective range beingextended greatly to the low input energy range depending on the value ofthe DC voltage Vb, thereby enabling a wide range control of gamma valueand providing an extended input energy latitude. Moreover, the contrastin the obtained output image is almost as excellent as that in Vb 0, asthe luminescence of the EL layer is little effected by DC voltage.

Further, control of the resistive element Re of the EL layerconcurrently varies the impedance of the EL layer. The contrast hithertolimited by the ratio of capacitances of the EL layer and PC layer, thusbecomes freely controllable by adjusting the resistive component Re,without depending on the geometrical structure of the PC layer and ELlayer.

In FIG. 2 which is showing schematically structure of a solid-stateimage plate embodying this invention and the manner in which theelectric power is supplied to the device, numerals 101 to 107 indicatethe constituting elements of the solid-state image plate, 101 beinglight-pervious support plate made of glass or the like, and 102 beinglight-pervious electrode, for example, made of metal oxide such as tinoxide. Numeral 103 indicates semiconductive electroluminescent layer ofapproximately 30 to 60 micron in thickness which comprises powder ofelectroluminescent material such as ZnSzCuAl and powder ofsemiconductive metal oxide such as Sn0 or Ti0 which has goodreflexibility against the luminescent spectrum of said EL material, saidpowders being binded by a vitreous material and formed in a layer. Thus,by the endowment of resistivity to an EL layer by mixing powder of aconcurrently reflective, resistive and semiconductive metal oxide, theluminescent output is effectively taken out from the EL layer withoutbeing absorbed by resistive powder. Further, the resistivity of the ELlayer can be easily controlled over a wide range by varying the amountof the resistive powder to be mixed. Accordingly, the matching of the PClayer to the load circuit including the EL layer can be easily attainedin the series connected resistive circuit, and a very effective controlof the AC photoconductive sensitivity of the PC layer is achieved by thecontrol of DC voltage, with the aid of usage of a PC layer containingpowdered photoconductive material. Thus, gamma value becomes widelycontrollable and the effective operating range of the intensity of theinput energy is extended.

Out of the resistive intermediate layers 104 and 105, the former issemiconductive reflecting layer of about 10 micron in thickness whichcomprises powder of a light-reflective and ferroelectric material suchas BaTiO and powder of semiconductive metal oxide such as Snt) or Tisaid powders being bonded with a vitreous material or a plasticmaterial. in this case, a vitreous bonding material is preferable formaking ohmic layer, while a plastic material is advantageous fornonohmic layer. Numeral 105 indicates impervious semiconductive layer ofabout micron in thickness which comprises, for example, black paintmixed with powder of nonlinear resistivity such as CdSzCl or powder oflinear resistivity such as carbon black and which is formed in a layer.By providing such resistive intermediate layers, dielectric breakdown ofEL layer by DC voltage or even by AC voltage is prevented. Moreover,resistivity of the series connected resistive load circuit including theEL layer can be adjusted by varying the resistivity of the intermediatelayers, so that the resistivity of said load circuit is set at anappropriate value of the same order as the dark resistivity of the PClayer or lower than that. Therefore, limitation for the resistivity ofthe EL layer is much relieved. For example, if the EL layer has beenproduced with extremely low resistivity, the intermediate layer 104 ismade so as to have an ohmic resistance asdescribed above, theresistivity being set at an appropriate value higher than that of the ELlayer, thereby to attain the matching of the load to the PC layer in theresistances. As described above, the intermediate layers allow easyfabrication of the resistive EL layer and eliminate the effect oflimitation of the resistivity to luminescent characteristics, andfurther facilitate easy matching of the DC resistances between theseries connected load circuit including the EL layer and the PC layer,thereby permitting very effective control of the AC photoconductivesensitivity of the PC layer by the DC voltage.

As mentioned previously, by selecting the resistivity of each relevantelement so that the transversal resistance of the DC load circuitincluding the EL layer is similar to or lower than the dark resistanceof the PC layer, the DC voltage across the PC layer is made high whenthe intensity of input energy signal is zero or very low. Thus, gammavalue and input-to-output characteristics in low input energy range areimproved, and the effective operating range of intensity of input energysignal is remarkably extended.

Further, as the intermediate layers are endowed with resistivity bymixing of powder of resistive material, the resistivity can be freelycontrolled by varying the amount of the powder over a wide range. Thismakes very easy the adjustment of the resistance of the series-connectedload circuit including the EL layer or the matching of resistancesbetween said load circuit and the PC layer, and accordingly facilitatesfabrication of the complete device, and further makes easy improvementof gamma value and the effective operating range of the intensity ofinput energy.

It will be noted that as the resistive intermediate layers containpowder of ferroelectric material such as BaTi0 the overall dielectricconstant of the layers is raised. This raised dielectric constant lowersAC voltage loss in the intermediate layers. This fact is anotheradvantage of the resistive intermediate layers, beside theabove-mentioned advantages.

Further, the ferroelectric material such as BaTill has a high specificresistivity. Therefore, when resistive binder material is used for theintermediate layer, the use of the ferroelectric material also makespossible controlling of resistivity of the intermediate layer by varyingthe amount of the material to be mixed, beside it serves to raise theoverall dielectric constant of the layer. On the other hand, in theintermediate layer the resistivity of which is presented by adding ofresistive powder, particles of highly resistive ferroelectric materialare intermixed with particles of resistive material, as theferroelectric material is also used. As the result, two dimensionaluniformity in the resistivity of the intermediate layer is improved, asthe condensation and maldistribution of the resistive powder are thusprevented.

Returning to FIG. 2, numeral 106 indicates the photoconductive layer orPC layer of about 200 to 500 micron in thickness, which is formed ofphotoconductive powder bound by plastics or a similar binding material,said photoconductive powder being a material which is sensitive not onlyto the visible light but to a radiation such as X-ray, infrared ray andultra-violet ray, such as, for example, cadmium sulfide activated withan element of IB group such as Cu or Ag and an element of VII B groupsuch as C1, the latter element of Vll B group being able to besubstituted by an element of III B group such as All or Ga. Numeral 107indicates electrode of vapourdeposited metal, for example, aluminum.This electrode is pervious not only to a radiation such as X-ray, but tothe visible light, and can be formed in a gappy pattern such asequispaced parallel lines, lattice or mesh. Numeral 108 represents inputenergy signal, which is not limited to the visible light, but can beother radiation such as ultra-violet ray, infrared ray or X-ray. Numeral109 indicates output visible image. Numerals 110 and 111 indicatevoltage sources for the solid-state image plate applied across theelectrodes 102 and 107, 110 being the AC operating voltage source and111 being variable DC voltage source. Among the above-described elements101 to 107, the most important one for realizing the above-mentionedcontrol by DC voltage is the semiconductive EL layer 103.

The resistivity of the EL layer is theoretically required to beappropriately lower than the dark value of resistive component of the PClayer; that is, to be in the semiconductive range of the order of 10 to10 ohm-cm, where the characteristics is fairly linear. However, thisrequirement for the resistivity presents several technical problemsincluding difficulties relating to construction and manufacturingmethod. The conventional techniques for imparting electroconductivity toa solid layer made of a highly resistive material such as plastic orglass, include a process of dispersing resistive material into theplastic or glass body. However, the resistivity obtained by such processis limited to one very near to that of a conductor or to one havingdirectivity, and a solid layer which has a resistivity of 10 to 10ohm-cm. belonging to the semiconductor range and which maintains ohmiccharacteristics up to a considerably high electric field, has not beenobtained. Though carbon black is comparatively satisfactory as aresistive material, it is not suitable for the material for impartingsemiconductivity to the EL layer, as it absorbs the luminescent lightfrom the EL powder. Use of metal powder such as Cu or Sn also presents adifficult problem, since such material is apt to be oxidized ordeteriorated at a high temperature expected in the manufacturing processand further, since pulverizin g of such material has a limitationbecause of its high malleability. Moreover, resistivity of such metallicmaterial is too low to allow easy control of the resistivity of thelayer. Especially, if a plastic binding material is used, the ohmicresistivity will be maintained by no means up to a high electric field,partly because of the inferior thermal property of the plastic.

In order to overcome such difficulties, the semiconductive EL layer 103as shown in FIG. 2 has been introduced by this invention. According tothis invention, the resistive material is selected from thesemiconductive metal oxides including Sn0,, W0 Sb O and Tit! which arestable at a considerably high temperature in the atmospheric environmentand readily available in the form of pulverized product, and which havehigh reflexibility to the light in the visible spectrum emitted from theEL powder. As the binder of the EL layer, is used a vitreous materialwhich is thermally stable up to a considerably high temperature and intowhich metal oxide such as Sn0 is preferably fusible in some extent, theohmic characteristics of the resistivity and thermal stability of theelectrical properties being taken into consideration. It is required toselect a vitreous binder the softening point of which is lower than thatof the support plate 101 and the heat expansion coefficient of which issubstantially the same as that of the support plate, so as to ensuresatisfactory application of the EL layer to the support plate. Ofcourse, the binder must be light-pervious, as it is used in the ELlayer. The control of the resistivity of the semiconductive EL layeraccording to the above-mentioned construction is made by varying thevolumetric ratio of the metal oxide powder in relation to the totalvolume. In this process, it is important to properly select the relativegradings of the resistive powder, EL powder and binding vitreous powder,in order to ensure sufficient adhesion among the resistive powder, ELpowder and support plate 101 and to obtain smooth layer. The followingtable 1 shows the gradings and volumic percentages of the ingredients ofthe above-described mixture: that is, Sn0 powder used as the resistivematerial, ZnS:CuAl powder as the electroluminescent material and thevitreous material as the binder. Table 2 shows an example of thecomposition of the vitreous binder. Table 3 shows volume expansioncoefficients and softening points of the vitreous binder and thelight-pervious support plate (a glass plate).

coefiicient point, C.

Vitreous binder X- Support plate (glass) As to the grading of thepowder, it is important that the diameter of the particles of vitreousbinder is always smaller than those of the other two materials. If thisrelation is reversed, the mixture will not unite. The percentage of Sn0powder can be varied in a range of 10 to percent, causing correspondingvariation in the resistivity. The heating temperature was set at 640 C.in this embodiment. The resistivity of thus obtained EL layer showed afairly good linearity in the semiconductor range of 10 to 10 ohm-cm.Moreover, thus obtained EL layer is highly resistive against heat andenvironmental conditions. The resistivity of the intermediate layers,that is, the semiconductive reflection layer 104 and the semiconductivenonpervious layer 105, is selected so that the total of the resistancesof the two intermediate layers and the EL layer is at most not higherthan the dark resistance (that is, resistance under no light input) ofthe PC layer 106. Generally, the resistance of the intermediate layersmay be approximately the same or lower in comparison with that of saidEL layer, and may have a nonlinear current-voltage characteristics.Thus, AC voltage loss in the intermediate layers is decreased by theabove-mentioned nonlinearity of resistance, thereby the AC voltage beingeffectively impressed on the EL layer. Further, drop of the distinctionin the output image due to dispersion of AC current in the intermediatelayers is prevented. Therefore, the nonlinearity of the characteristicsis rather preferable, when satisfactory matching between the resistanceof the EL layer and the dark resistance of the PC layer is obtainablewithout adjustment of the resistance of the intermediate layers.

It is important that softening point of the vitreous binder used in theintermediate layers is lower than that of the vitreous binder used inthe EL layer, and that volume expansion coefficients of the two bindersare approximately the same.

In the above-described constitution of the device of this invention, atleast either one of the semiconductive reflection layer 104 and thesemiconductive nonpervious layer 105 can be omitted. If the nonperviouslayer 105 and further the reflection layer 104 have been removed, theluminescent output from the EL layer 103 can be fed back to the PC layer106. Accordingly, if the spectral characteristics of the EL material andthe PC material are selected so that the PC layer effectively respondsto the light from the EL layer, sensitivity of the device to the inputenergy will be improved and at the same time, gamma value can be raisedappropriately under such improved sensitivity. In such case, the ELlayer 103 can be made in the form of a compound layer consisting of alayer for displaying output image and a layer for feedback of the light,with a semiconductive nonpervious layer therebetween.

FIG. 3 shows input versus output characteristics of the embodiment shownin FIG. 2, as plotted on a logarithmic chart, where the AC operatingvoltage is fixed at 450v., its frequency being 1 kc. and the DC voltageis varied, as the parameter, from zero to 400v. In the diagram, theintensity of input energy signal is represented by dose rate ofcontinuous X-ray from a 113 kVP X-ray tube. As is seen from the diagram,gamma value is variable widely and continuously in a range of l to 3,and input energy latitude is extended by nearly one hundred times of theconventional value.

As described above, by giving resistivity to the EL element and otherappropriate elements, superimposing a DC voltage on the AC voltageapplied to the series connected EL element and PC element, and providingmeans for varying said DC voltage, the AC photoconductive sensitivity ofthe PC layer in relation to the intensity of input energy can becontrolled, and characteristics in low input energy range, gamma valueand effective operating range for the input energy intensity can beadjusted widely and continuously.

Further, in the above-described example, if the DC voltage issuperimposed in such a manner that the electrode in the EL element sideis of positive polarity and the electrode in the PC element side isnegative, the above-mentioned improvements are realized with much highersensitivity or effectiveness in comparison with the case where the DCvoltage is applied in opposite direction.

Accordingly, in the above embodiment, if means for changing the polarityof the DC voltage Vb, for example, a changeover switch is provided tothe voltage source, the operating characteristics can be varied only bychanging the polarity of the DC voltage without varying magnitude of thevoltage. Therefore, a wide variability of the operating characteristicsis obtained by providing means for controlling at least either one ofmagnitude or polarity of the DC voltage to be superimposed on the ACvoltage.

Now, turning to the PC layer 106, it will be noted that thephotoconductive material CdSzCuCl is essentially less sensitive to theradiations such as X-ray than to the visible light. In order to improvethe sensitivity to such radiations, an appropriate amount of radiationluminescent fluorescent powder (for example, orange luminescentZnCdSzAg) is added to the photoconductive powder of CdSzCuCl, and themixture is bound by a plastic binder. By such composition, thesensitivity is increased by more than ten tifies of the original value.This is because the radiation luminescent fluorescent material isexcited by the incident radiation (for example, X-ray) at the same timewhen the PC material is excited, and the PC material is further excitedby the visible light converted by the radiation luminescent material. Asthe utility factor of the converted visible light corresponds to 41:-solid angle, increase of the sensitivity is remarkable. For example, anexperiment with a PC layer containing CdSzCuCl added with 10 volumicpercents of ZnCdSzAg, showed that the characteristic curves in FIG. 3were shifted laterally to lower input range by one decimal scale by saidaddition of ZnCdSzAg. Especially, in this case, resistivity anddielectric strength of the fluorescent powder show much higher values incomparison with those of the PC powder under no input light. That is, itwill be seen that the addition of the fluorescent powder improves darkresistivity and dielectric strength of the PC layer.

As described above, according to this invention, operatingcharacteristics of the energy-responsive luminescent device is improvedparticularly in low input range, and gamma value of the solid-stateimage plate becomes widely variable, and further the input energylatitude is remarkably extended, by imparting resistive impedances tothe EL layer and the required intermediate layers interposed between theEL layer and the PC layer with unique constitution and manufacturingmethod and by controlling the DC voltage superimposed on the ACoperating voltage.

Further, the EL layer of this invention is formed from a mixture whichcontains powder of vitreous material, powder of BL fluorescent materialand powder of at least one lightreflective and semiconductive metaloxide selected from group containing Sn Ti0 W0 and Sb 0 the mixturebeing heated to fuse the vitreous material. Thus, the EL layer of thisinvention presents an ohmic resistivity which is well stable even inhigh voltage range and a highly efficient EL luminescence, withoutpossible decrease of the luminescent efficiency.

We claim:

1. A solid-state energy-responsive luminescent device comprising anelectroluminescent layer which is excited to luminescence by an ACvoltage applied thereto, a photoconductive layer provided on saidelectroluminescent layer, the AC impedance of said photoconductive layerbeing variable depending on the intensity of an incident energy and saidAC impedance in a dark state being higher than the AC impedance of saidelectroluminescent layer, a pair of electrodes sandwiching thecombination of said two layers therebetween, at least that one of saidelectrodes which is disposed on the electroluminescent layer beinglight-pervious, and means for applying an AC voltage and a DC voltagesuperimposed on said AC voltage across said layers by means of saidelectrodes, whereby the AC voltage across said electroluminescent layer,and therefore the luminescent output, is controlled substantiallyaccording to the variation of the AC impedance of said photoconductivelayer due to the variation in the intensity of the incident energythereto, wherein said photoconductive layer comprises a powder ofphotoconductive material bound with a binding material so that thephotoconductive sensitivity of said photoconductive layer under an ACvoltage is substantially increased as the superimposed DC voltage isincreased and so that the DC resistance of said electroluminescent layeris lower than the DC resistance of said photoconductive layer in a darkstate so that the DC voltage across said photoconductive layer in therange of low incident energy is high enough to enhance thephotoconductive sensitivity of said photoconductive layer.

2. A solid-state energy-responsive luminescent device as defined inclaim 1, wherein said electroluminescent layer is composed of powder ofelectroluminescent fluorescent material and powder of resistive metaloxide which has reflective power against the light from said fluorescentmaterial,

said two powders being mixed with a vitreous medium.

3. A solid state energy-responsive luminescent device as defined inclaim 2, wherein the specific resistivity of said electroluminescentlayer is in a range of 10'' to l0 ohm-cm.

4. A solid-state energy-responsive luminescent device as defined inclaim 3, wherein said resistive metal oxide is at least one selectedfrom a group including Sn0 Ti0 W0 and Sb 0 5. A solid-stateenergy-responsive luminescent device as defined in claim 1, wherein atleast one resistive intermediate layer is interposed between saidelectroluminescent layer and said photoconductive layer, and thetotal-DC resistance of the combined layer comprising saidelectroluminescent layer and said intermediate layer is lower than theDC resistance of said photoconductive element in a dark state.

6. A solid-state energy-responsive luminescent device as defined inclaim 5, wherein said resistive intermediate layer is a compound layerconsisting of a light-reflective layer positioned on theelectroluminescent side and an impervious layer positioned on thephotoconductive side.

7. A solid-state energy-responsive luminescent device as defined inclaim 2, wherein one of said pair of electrodes comprises light perviousconductive film of metal oxide deposited on a light pervious glasssubstrate.

2. A solid-state energy-responsive luminescent device as defined inclaim 1, wherein said electroluminescent layer is composed of powder ofelectroluminescent fluorescent material and powder of resistive metaloxide which has reflective power against the light from said fluorescentmaterial, said two powders being mixed with a vitreous medium.
 3. Asolid state energy-responsive luminescent device as defined in claim 2,wherein the specific resistivity of said electroluminescent layer is ina range of 107 to 109 ohm-cm.
 4. A solid-state energy-responsiveluminescent device as defined in claim 3, wherein said resistive metaloxide is at least one selected from a group including Sn02, Ti02, W03and Sb205.
 5. A solid-state energy-responsive luminescent device asdefined in claim 1, wherein at least one resistive intermediate layer isinterposed between said electroluminescent layer and saidphotoconductive layer, and the total DC resistance of the combined layercomprising said electroluminescent layer and said intermediate layer islower than the DC resistance of said photoconductive element in a darkstate.
 6. A solid-state energy-responsive luminescent device as definedin claim 5, wherein said resistive intermediate layer is a compoundlayer consisting of a light-reflective layer positioned on theelectroluminescent side and an impervious layer positioned on thephotoconductive side.
 7. A solid-state enerGy-responsive luminescentdevice as defined in claim 2, wherein one of said pair of electrodescomprises light pervious conductive film of metal oxide deposited on alight pervious glass substrate.